Linear drive device

ABSTRACT

The linear drive device has two motors (M 1 ,M 2 ) firmly arranged relative to each other, one driving the nut (10), the other driving the screw spindle (20), whereby the nut (10) driven by motor (M 1 ) is mounted axially firmly relative to the motor (M 1 ), whereas the screw spindle (20) is driven by motor (M 2 ) through the intermediate of a non-rotating, but axially shiftable coupling element (21).

BACKGROUND AND SUMMARY OF INVENTION

The invention relates to a linear drive device with two motors firmlyarranged relative to each other, one driving a nut, the other driving ascrew spindle. Such drive devices with two firmly arranged motors areknown from Swiss Pat. No. 647,305. In these known drive devices thescrew spindle is always arranged rotatably, but axially fixed, whereasthe axial displacement movement of the nut is led to the exterior of thedevice. This type of construction has been chosen in order to obtain afavorable relationship between the maximum stroke and overall length.

If such a linear drive device is designed to perform fast movements witha small stroke, which can be achieved by electronic control of themotors, the mass to be moved is important. According to the inventionthe moving mass can be reduced by holding the motor-driven nut axiallyfirmly, whereas the other motor drives the screw spindle by means of anon-rotatable but axially movable coupling element.

It is advantageous to use in a drive device with a small stroke acoupling element having a rigid driving part and a membrane serving aspower take-off part connected therewith. By means of such a couplingelement, which allows but only a small stroke, the axially moving massis further reduced because only the membrane is deformed.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing a few embodiments of the invention are representedsimplistically:

FIG. 1 shows an axial section of a drive device with axially alignedmotors;

FIG. 2 shows an axial section of a drive device with juxtaposed motors;and

FIGS. 3 and 4 show axial sections of drive devices with axially alignedmotors of which one has a hollow shaft.

DETAILED DESCRIPTION OF THE DRAWING

The linear drive device according to FIG. 1 functions with two motorsarranged in a housing G. Motor M1 drives a nut 10, whereas motor M2drives a screw spindle 20. The nut 10 is axially firmly, but rotatablymounted. To this purpose it is connected firmly with the shaft of motorM1 and additionally supported axially by means of the axial bearing 11.

Motor M2 drives the spindle 20 by means of an axially movable couplingelement which is designated with the numerals 21 as a whole.

In the represented embodiment this coupling element consists of aplatelike driving part 22 connected with the shaft of motor M2 and anannular, undulated membrane 23 attached at the edge of part 22. Themembrane serves as driving part and is provided with a centrallydisposed hub which establishes the connection to the screw spindle 20.To the extent of the stroke permitted by the elastic deformation ofmembrane 23, the spindle 20 is therefore axially movable.

The axial displacement or stroke of the spindle occurs with simultaneousrotation thereof. This movement is led to the exterior of the housingwith the aid of an axial bearing 24 in a sled 25 which is axially guidedby means of bars connected exteriorly with a traverse 27.

As results from the previously described construction of the drivedevice the membrane allows only a relatively small stroke of thetraverse 27. Of course the stroke could be increased by use of acoupling element with a coupling sleeve and an axially movable corepiece therein. However, this would increase the mass of the parts to bemoved, which is a disadvantage for rapid reciprocating motions.

The drive device is used namely in a machine tool for movement of aworking tool with variable stroke and variable frequence. According tothe invention the number of revolutions per time unit is controlledelectronically relatively to each other, according to the workingpattern.

An important field of use is the superfinish of internal combustionengine pistons which are superfinished according to a certain workingpattern. Its originally exclusively cylindrical form is transformed intoa slightly embossed and at the same time not circular form. If thissuperfinish is performed on a lathe, the lathe tool has to be movedto-and-fro at each revolution of the work piece by a small amount.

For such a superfinish the stroke of the lathe tool is 0.1 to 1.0 mm.

The movement, however, must be exactly synchronous to the number ofrevolutions of the work piece to be finished. Such a superfinishingpattern which is synchronous to the number of revolutions of the workpiece may be achieved by electronic control of the number of revolutionsof motors M1 and M2 of the drive device. The working pattern may bestored electronically, e.g. on tape or disk. The traverse 27 bears thena diamond charged lathe tool D.

For such a control both motors are operated at the same speed in thesame sense of rotation and the electronic control produces a very smallchange in the number of revolutions of one of the two motorsrespectively. Depending on the increase or decrease of the speed thetraverse 27 is moved either forward or backward.

The drive device with two motors according to the invention has the bigadvantage over the known drive devices that the full drive force isavailable at each movement of the traverse, since both motors areoperated with almost normal speed and therefore full power.

As the feed should work free from play, but nevertheless with thesmallest friction as possible, the nut 10 may consist of a ball rotatingnut. The guide bars 26 of the sled may be protected against dust bycollars.

Instead of axially aligning the motors as shown in FIG. 1, they may bearranged side by side, as shown in FIG. 2. In this embodiment the nut 30is mounted axially firmly but rotatable in the housing G and connectedwith a toothed belt disk 31, allowing motor 1 to drive the nut. Motor M1is provided to this purpose with a toothed belt disk 32 and drives nut30 by means of the toothed belt 33.

Motor M2 drives the screw spindle 40 by means of a coupling element 41.The design of the coupling element 41 corresponds to that of the abovedescribed coupling element 21. The screw spindle extends with a smoothpart of the spindle 42 to the exterior and moves by means of an axialbearing 43 a sled 44 which is guided by the guide bars 45 arrangedoutside the housing.

FIGS. 3 and 4 show two further embodiments of the linear drive device inwhich the smooth part of the spindle extends through the hollow shaft ofone of the two motors to the exterior. The cross hatching indicates thatthe motor is represented in a sectional view.

In the drive device according to FIG. 3 motor M2 drives the screwspindle 60 by means of the coupling 61, said screw spindle having asmooth part 62 extending through the hollow shaft 51 of the motor M2 tothe exterior.

Motor M2 drives the nut 50 by means of its hollow shaft 51, said nutbeing mounted axially firmly but rotatable at 52. In this embodiment amilling head is arranged at the outer end of the spindle as workingtool, instead of a lathe tool. This simplifies the construction, becausethe additional axial bearing of the sled and its guides is madesuperfluous.

If there is no feed of the milling head, both motors M1 and M2 arerotating at identical speed. An advance or withdrawing of the millinghead F is achieved by a slight increase or decrease of the speed of oneof the two motors. This means that only with the respective advance orwithdrawing of the milling head there is a small rotation of the spindle62 relatively to the hollow shaft 51.

The drive device according to FIG. 4 differs from that according to FIG.3 by the fact that in this embodiment motor M2 is driving the spindle 80by means of a hollow shaft 81 and a coupling element 82. Motor M1 drivesthe nut 70, which is mounted axially firmly by means of the bearing 71.The smooth part of the spindle 83 extends through the hollow shaft 81and displaces sled 85 by means of an axial bearing 84 arranged therein.The sled 85 is mounted slidingly on the guide bars 86.

It is clear that the spindle 83 can be provided, as in the drive deviceaccording to FIG. 3, with a milling tool instead of moving a sled. Thesame applies to the drive device according to FIG. 2.

Finally it should be mentioned that all of the represented linear drivedevices may be provided with a coupling element with a coupling sleevecontaining an axially movable core piece. Such a coupling element allowslarger axial displacements compared to a coupling elment with amembrane.

It appears that the double motor design with axially firmly butrotatably mounted nut and axially movable screw spindle is not suitablefor linear drive devices with larger stroke, due to the excessiveoverall length.

I claim:
 1. A linear drive device comprising a housing;first and secondmotors mounted on said housing; an internally threaded nut; firstcoupling means for coupling the output of said first motor to said nutto rotatably drive said nut; an externally threaded spindle threadedlyengaging said nut; second coupling means for coupling the output of saidsecond motor to said spindle for rotatably driving said spindle; meansfor restraining only one of said nut and spindle against axial movement,the other of said nut and spindle being capable of axial movement; oneof said first and second coupling means including a shaft substantiallycoaxial with said spindle and axially spaced therefrom and a couplinginterconnecting said shaft and said axially movable one of said nut andspindle for transferring rotational forces from said shaft to saidaxially movable one of said nut and spindle while permitting limitedaxial movement thereof so that the axial position of said movable one ofsaid nut and spindle can be adjusted by controlling the relativerotational speeds of said first and second motors.
 2. A drive deviceaccording to claim 1 and further a linearly movable means for couplingsaid axial movement to the exterior of said housing.
 3. A drive deviceaccording to claim 2 wherein said coupling comprises a generallycup-shaped substantially rigid member fixedly connected to said shaftand an axially flexible membrane attached across said rigid member andconnected to an end of said movable one of said nut and spindle.
 4. Adrive device according to claim 3 wherein said linearly movable meanscomprises first and second guide bars slidably movable in said housing,a sled rigidly connected to said bars within said housing, thrustbearing means interconnecting said sled and said axially movable one ofsaid nut and spindle, and a tool connected to said bars outside of saidhousing.
 5. A drive device according to claim 4, wherein said motors arecoaxially mounted in said housing.
 6. A drive device according to claim4, wherein said motors are mounted so that the output shafts thereofextend along parallel, laterally spaced axes.